Sampling the alpha channel of a texture does not require alpha blending to the framebuffer to be enabled. Make sure that your texture actually does have an alpha channel, and it's not all 1.f in each pixel.

Try debugging visually the contents of the alpha channel with the following fragment shader:

ok I found out was the problem.
First it was caused by clearing texture, which is used to apply shaders on whole screen rather that on parts that are drawn (This is I think SFML problem).
Second, I had to normalize position, so it looks like this now